In succession to conventional installations for agglomeration of minerals on a conveyor, in which the agglomerated mineral was discharged from the conveyor at the end of the firing, that is to say, at its maximum temperature, in order to be cooled in a separate installation, conveyors are now employed with integrated cooling. In these new installations the firing zone is followed by a cooling zone in which cold air passes through the agglomerated mass from top to bottom, the air being drawn through by suction windboxes arranged below the conveyor.
In a conventional agglomeration conveyor having separate cooling it is important to regulate the position of the firing point correctly. The firing point is to be understood as the point at which the flame front, starting from above the layer of material where ignition takes place, reaches the bottom portion of the layer, combustion then being complete throughout the whole thickness of the layer. If the duration of the firing is reduced by acceleration of the propagation of the flame front through the layer, the productive capacity of the conveyor is no longer employed at its maximum since one would have been able to obtain a greater flow of material by increasing the speed of the conveyor. Conversely, increase of the duration of the firing leads to the firing being imcomplete at the end of the conveyor, which increases the proportion of unfired, and hence rejected, fines and reduces the quality of the product provided. Hence, there is a definite economic interest in keeping the firing point at a specified point, while keeping the conveyor speed at its maximum.
In an agglomeration conveyor installation with integrated cooling, the object of the regulation is not, properly speaking, to keep the firing point at a predetermined point but to obtain agglomerated products leaving the conveyor at a given temperature and cooling air at a temperature which is not dangerous to the cooling fans and filters, that is to say, a temperature less than about 250.degree. C. In short, too high a temperature of the products at the output from the conveyor brings about difficulties in handling them and the conveyor belts will be impaired by masses of the product which are still red hot. Conversely, if the temperature is too low the useable output of the installation will be reduced because part of the energy brought into play in the cooling zone will have been used to no avail.
But in fact one is also led to look, even in installations with integrated cooling, for regulation of the position of the firing point. That is, if the firing point shifts into the cooling zone the temperature of the cooling air becomes excessive and is incompatible with the characteristics of the cooling fan. In addition, because the fan is a volumetric system, an increase in temperature reduces the mass flow of air and hence its cooling capacity, which causes yet another supplementary increase in temperature. It only remains then to stop the process and cool the product on the stationary conveyor to the detriment of production.
If, on the contrary, the firing point shifts towards the ignition hood with shortening of the firing zone, part of the flow from the firing fan will be employed to do cooling. A new position of stability of the firing point will develop because the flow of gas will fall in the firing zone in the same way as the vertical speed of the flame front in the layer. But this will reduce the agglomeration and cooling performance since the transfer of heat from the material to the gases is proportional to the masses present. For the firing, the speed of flow of the air is then too low and the firing zone spreads, the flame front and the heat front separate too far and the quality of agglomeration and the fines are disturbed. Hence it is preferable, whatever the required production, to keep the firing point in its normal position, that is to say, so that the firing fan acts only in the firing zone and the cooling fan acts only to cool the agglomerated products.
Whether it is a question of finding an optimum temperature of the products discharged or a stable position of the firing point, one runs into great difficulty in checking these operating conditions satisfactorily.
A graph of the temperature of the agglomerate at the output from the conveyor is not continuous but may display a seccession of peaks corresponding with compact and impermeable incandescent fragments of agglomerate which have had difficulty in being cooled. Such measurement of the temperature of the agglomerate is therefore useful but difficult to use in automatic control.
It is equally difficult to determine the position of the firing point by measurement of the temperatures in the windboxes in the firing zone. Actually, the temperature measurements in the various windboxes give an apparently continuous curve which develops when the firing point shifts. But this curve is difficult to analyze by a computer and the actual shape of the flame front may be disturbed by differences in permeability of the layer between the middle and the edges of the installation. In addition, thermal inertia is very large in the firing zone, particularly due to the heat absorbed in evaporating the water contained in the mixture, with the result that even a large shift in the firing point is expressed only as a small variation in the measured temperatures.
According to the present invention, there is provided a method of regulating the process of agglomeration of a mineral on an endless conveyor, the mineral on the conveyor passing through a firing zone provided with suction windboxes under the grate connected to a common suction duct, and then a cooling zone in which the fired product is cooled by passing air through the layer of fired agglomerated products using suction windboxes connected to a common suction duct, the method comprising measuring the temperature of the gases in the common suction duct from the windboxes in the cooling zone, comparing the measured temperature with a preset temperature, and using a signal resulting from this comparison to act upon the flow of suction gases from the windboxes in the firing zone.